Wood Beam (LRFD)

How it calculates

The Wood Beam (LRFD) calculator checks sawn lumber, glulam, LVL, PSL, LSL and I-joist beams to NDS 2024 Load and Resistance Factor Design. Beams can be single or multi-ply, with optional steel flitch plates. You enter unfactored loads by type, and the calculator builds the factored load combinations.

Six checks

You get six utilization ratios. A beam passes when every ratio is 1.0 or less.

• Bending: M_u / φM_n
• Shear: V_u / φV_n
• Bearing: R_u / φR_n at the most critical support, with minimum bearing lengths for end and interior supports
• Short-term deflection: under L + Lr, S or 0.42W
• Long-term deflection: under kD + L + Lr, where k adds creep to the dead load (0.5 dry, 1.0 wet)
• Dead plus live or snow: D + (L or S)

Deflection uses unfactored loads. Each limit is a span ratio (L/n) capped by an absolute limit you set. Cantilever limits can be doubled.

Factored resistance

Each resistance converts the ASD reference value with the format conversion factor K_F, then applies the resistance factor φ:

• Bending: φb = 0.85, K_F = 2.54
• Shear: φv = 0.75, K_F = 2.88
• Bearing: φc = 0.90, K_F = 1.67 (2.88 for I-joists)

Bending and shear also take the time effect factor λ. Bearing does not.

Time effect factor

λ is set per load combination:

• 0.6 for dead load alone
• 0.8 for occupancy live load by default, adjustable by live load type
• 0.8 for snow, roof live and rain
• 1.0 for wind and seismic

The governing bending combination is the one with the highest M_u / λ. A smaller moment can govern if it comes with a longer-duration load.

Adjusted bending stress

F'b combines φb, K_F and λ with the factors for the product you pick:

• Sawn lumber: F'b = φb × K_F × λ × Fb × CM × Ct × CL × CF × Cfu × Ci × Cr
• Glulam, strong axis: F'b = φb × K_F × λ × Fb × CM × Ct × Cfu × the lesser of CL and CV
• LVL, PSL and LSL: with beam stability (CL) and volume (CV) factors
• I-joists: the manufacturer's moment capacity

Moment capacity is φM_n = F'b × S, where S covers all plies. F'b already includes φb.

Beam stability

CL comes from the critical buckling moment of each unbraced segment, using the AWC TR14 method. You choose continuous bracing on the top face, the bottom face, both or neither. Bracing both faces removes lateral-torsional buckling, so CL = 1.0.

Shear, bearing and flitch plates

• Shear demand is taken at the highest point along the span, not at distance d, which is conservative.
• Bearing under point loads is not checked.
• Flitch beams use the transformed timber and steel section for bending. Shear and bearing are assigned to the timber.
• Multi-ply beams bending about the weak axis act non-compositely.